Electrical type strain gauges are based upon the measurement of an inherent electrical property (commonly capacitance or resistance) which is a function of an induced strain. One of the simplest types of strain gauges uses the property of piezoresistivity, or a change in resistance which is proportional to an induced strain (not to be confused with piezoelectricity, which is an induced electrical charge when the material is strained). Piezoresistive films are used to form macroscopic strain gauges (for measurement of many types of structural deformations), or are patterned to small geometries to form micromachined devices which can function as force transducers, such as accelerometers or pressure sensors.
The magnitude of the piezoresistive effect is quantified by a gauge factor (K), which is a proportionality factor between a relative resistance change (.DELTA.R/R) and a strain (.DELTA.L/L) induced in the film by an applied stress. EQU .DELTA.R/R=K(.DELTA.L/L). (1)
Many commercially available thin-film piezoresistive strain gauges are made of metal foil which has been deposited onto a flexible polymer backing. These strain gauges are adequate for measuring strain in structures such as bridges, buildings, machine parts, etc. However, for certain other applications, such as measuring strains in biological tissues such as ligaments, or for measuring large movements in robotic applications, metal film strain gauges are inadequate because their flexibility is limited by material fatigue. An additional problem with metal film strain gauges is delamination of the gauge from the object to be evaluated.
Low inherent resistivity of metal film strain gauges poses more application problems. In order to achieve a strain gauge resistance which is large enough to measure and distinguish from lead wire resistance, several approaches are used. The metal films are designed to have a large length/width ratio of the metal line itself (as great as 500) to increase the resistance. The metal pattern is folded back upon itself so as to take up as little space as possible, but the size of the overall strain gauge is dictated by the required length/width ratio. Commercial metal film strain gauges have a typical resistance of 50-100 ohms. The contribution of lead wire resistance to the total strain gauge resistance can be appreciable, and introduces errors of several percent. Lead wires have an appreciable temperature coefficient of resistance, which makes compensation for the lead wire resistance difficult. To eliminate the effect of lead wire resistance, a multiple-terminal resistance measurement is recommended by strain gauge manufacturers, but this method of compensation is less desirable because more than two leads must be connected to the device.
For micromachined sensors, silicon is usually the material of choice because of its compatibility with semiconductor processing. The use of silicon limits the choice of substrate, as either single crystal substrates or else a substrate which can tolerate silicon deposition temperatures must be used. Silicon cannot be used for larger-scale applications because of its limited flexibility.
Polymer based strain gauges have been suggested as a more flexible substitute. These gauges, for the most part, have been produced by imparting piezoresistivity to non-conductive, organic polymer insulating phase material. The insulating phase is made conductive by heterogeneously intersticing or imbedding conductive material into the insulating matrix whereby at high enough loading of conductor, contact between grains allows current flow. Presumably, changing the distance between conductive particles by expanding or contracting the film changes the conductivity, resulting in a piezoresistive effect.
United Kingdom patent GB2141548A by Welwyn Electronics Ltd. discloses a transducer incorporating an electrical resistance strain gauge element in the form of a conductive polymer comprising a dispersion of electrically conductive or resistive particles in an electrically insulating organic polymer. The particles comprise a dispersion of electrically conductive carbon in an organic polymer such as epoxy, alkyd, polyester, acrylic or silicon materials or copolymers thereof. It was necessary to provide a primary member, adapted to be deflected, comprising aluminum or aluminum alloy because the heat treatment which was required to impart resistivity to the gauge degraded the elastic properties of certain materials on the strain gauge, making them unsuitable for use by themselves.
Also, in the 1991 proceedings of the ISHM, Rojek, et al., reported a three-component piezoresistive film where tin and graphite particles were required to be intersticed in the organic resin polyesterimide (polimal).
In an article entitled "Polymer Thick-Film Technology: A Possibility to Obtain Very Low Cost Pressure Sensors?," published in Sensors and Actuators A, 25-27 (1991), pages 853-857, carbon loaded organic polymer thick-film resistors were used as pressure sensors under limited temperature conditions.
In the 1992 proceedings of the Materials Research Society (April, 1992), Frazier, et al., published a report entitled "Mechanical and Piezoresistance Properties of Graphite-filled Polyimide Thin Films." The piezoresistance coefficient was a function of graphite loading, with good piezoresistive properties exhibited in the loading range of 15% to 25% graphite.
The piezoresistive properties of strain gauges made from such materials have a number of shortcomings. First of all, the particulate nature of the films prevents small geometry patterning. A second problem is delamination of the polymer from the particle surface. This is a common problem with carbon matrix composites for structural applications, and it is commonly attributed to poor wetting and poor adhesion of the filler to the matrix polymer. A similar failure mechanism occurs in carbon-filled polymers which are used for conductive polymer films, especially when they must undergo large and repeated deformations. Therefore, as piezoresistivity increases in the 15% to 25% graphite loading range, the structural integrity of the gauge becomes less stable.
A further disadvantage of filled systems is that it is often difficult to keep uniform dispersions of carbon and metal polymer mixtures. Shelf stability of the carbon-matrix dispersion can be poor due to settling out of the filler particles. If the films are compounded from dry ingredients, it is often difficult to form a uniformly compounded product. Such strain gauges are reported to have non-uniform resistances associated with their heterogeneous nature. (See B. E. Roberston and A. J. Walkden, "Tactile Sensor System for Robotics," in Robot Sensors, Vol. 2--Tactile and Non-Vision, Alan Pugh, Ed., Springer-Verlag, IFS Ltd., UK, 1986.)
In U.S. Pat. Nos. 4,708,019 and 4,808,336, Rubner disclosed polymeric pressure transducers made from a piezoresistive blend of an iodine doped acetylene polymer in combination with an elastomer. While such strain gauges have a high degree of flexibility, the methods by which they are produced (polymerizing the polyacetylene into a film or elastomer which is deposited on the inside surface of the polymerization flask, or else polymerization into solvent-swollen rubber) cannot be used to coat on various substrates. There remains the disadvantage that such polyacetylene films are highly unstable. In the journal Molecular Crystals and Liquid Crystals (1985), Vol. 118, pages 129 through 136, the article "Electrical Conductivity of Modified Polyacetylenes and Polypyrroles" clearly indicated that no stabilizing effect to air could be obtained in iodine doped polyacetylenes, and the conductivity was highly sensitive to oxygen. As discussed in the article entitled "Electrical and Photovoltaic . . . ," the conductivity of iodine-doped polyacetylene falls off rapidly if the polymer is heated. This thermal instability would prevent the lithographic patterning of these films, as solvent-removal bakes are a necessary part of the photoresist patterning process.
For these prior art materials, the magnitude of the gauge factor (K) is approximately 2 for metals, 100-200 for silicon, 2-17 for polymer films with conductive filler, and 1.3-31 for doped acetylene polymer polymerized into a polymer matrix.
The discovery of a homogeneously conductive polymer which could be made piezoresistive without embedding carbon or metal conductive islands and without problems associated therewith, and without sacrificing thermal stability so as to permit submicron photolithographic featuring on strain gauges which are coated onto virtually any substrate, would be a welcome improvement in the art and an unexpected advantage.
The formation of homogeneous, conductive polymer films has been previously demonstrated. These films are composed of polymer chains which are soluble and processable in the conductive state (U.S. Pat. No. 5,262,195), or polymer films which have been made conductive by ion implantation (R. E. Giedd, M. G. Moss, M. M. Craig, and D. E. Robertson, "Temperature Sensitive Ion-Implanted Polymer Films," Nuclear Instruments and Methods in Physics Research B59/60, pp. 1253, 1991,) and are not binder-filler mixtures. Strain gauges formed from piezoresistive, homogeneous conductive polymer films would have several advantages over the prior art. The processing advantages of the carbon- or metal-based composites would be maintained (ease of coating on multiple substrates). The need for three- or four-terminal measurements which are necessary in the case of metals would be eliminated because of the polymer's higher resistance. Finally, a conducting polymer film would have greater uniformity because of its homogeneity. The conducting polymers would combine the high performance of silicon films with the processing capability of deposition on a wider variety of substrates.
It is not immediately apparent that a homogeneous, conductive polymer film would have a piezoresistive gauge factor which would be on the order of those of the prior art materials, as the conducting polymer consists of intertwined "wires" of polymer whose diameters are much smaller than the particles in carbon and metal matrix composites. Similarly, neither type of conductive polymer possesses the crystal structure inherent in silicon nor its band gap mechanism of conductivity. Likewise, the conductivity of the conductive polymer is not metal-like, as demonstrated by its resistance-temperature behavior. Nevertheless, we have shown that thin films of conducting polymers have gauge factors which are on the order of the prior art, and these materials can be used as strain gauges with many processing advantages over the state-of-the-art. Surprisingly, the gauge factors change very little with respect to temperature over a broad temperature range, although temperature compensation must be applied to correct for changes in absolute resistance.
It is therefore an object of the present invention to provide an improved polymeric strain gauge from homogeneously conductive polymers having improved bulk resistivity for both macro and micro sized geometric features.
It is a further advantage of the present invention to provide piezoresistive conductive polymers without having to load particulate carbon or metal materials into an insulating matrix.
It is a further object of the present invention to provide improved piezoresistive film gauges which can be patterned by photolithography and yet remain thermally stable.
It is a final principal object of the present invention to provide more flexible strain gauges which can have features as small as the limits of the patterning process detailed into the strain gauge while providing improved performance through improved substrate compatibility.